As known, an optical cable typically comprises an optical core and an external sheath enclosing the optical core. The external sheath is typically made of a polymeric material and has the primary function of protecting the optical core from the mechanical stresses.
The optical core of these type of cables typically comprises one or more optical modules, the term “optical module” indicating a fiber bundle—comprising one or more optical fibers—and a retaining element (e.g. a buffer tube or a tape) surrounding the optical fibers. The retaining element carries out the function of retaining the optical fibers and, optionally, protecting them against mechanical stresses.
An optical fiber is typically constituted by a glass core, a glass cladding and a single or dual coating layer.
The optical fibers may be arranged in various ways within the cable.
For instance, the so called “multi-loose tube cables” (briefly, MLT cables) typically comprise multiple optical modules (e.g. 3, 4, 6 or more optical modules), each module typically grouping a number of optical fibers in a buffering tube. The buffering tubes, typically made of a polymeric material (e.g. polybutylene terephthalate—PBT), are stranded about a central strength member, which is usually made of compacted steel strand or “glass reinforced plastic” (briefly, GRP). A binder or tape may also be provided around the optical modules for retaining them. The external sheath surrounds all the optical modules to provide environmental protection.
In MLT cables, the loads to which the cable is subject (tensile loads, bend loads, environmental loads) are mainly withstood by the central strength member and, optionally, by peripheral strength members arranged about the optical modules.
Besides MLT cables, other arrangements for optical fiber cables are known.
For instance, U.S. Pat. No. 4,659,174 discloses an optical cable element including an optical waveguide loosely embedded in a bundle of strength fibers, and a protective mantle surrounding the waveguide-containing bundle of strength fibers. The optical waveguides and the strength fibers surrounding them fill 50% to 90% of the space inside the mantle.
EP 1 324 090 discloses a central tube cable comprising buffer tubes housing optical fibers, the tubes being stranded around a bundle support member. The cable further comprises a cable jacket with strength members embedded therein.
EP 0 256 704 discloses an optical fiber cable comprising an outer sheath, four or five strength members and at least one optical fiber package accommodated in an interstitial space formed by the strength members.